1. Field of the Invention
The invention relates to a coordinate measuring device and to a method for operating a coordinate measuring device, according to the precharacterizing clauses of the corresponding independent patent claims.
2. Description of Related Art
In order to measure the position of moving target points, coordinate measuring devices are used, often so-called laser trackers. The term laser tracker is intended to mean devices which comprise at least one distance meter operating with a focused laser beam (referred to in the following description as a measurement beam). With the aid of a mirror which can be rotated about two axes, for example, the direction of the measurement beam is adjusted to the target point and recorded by angle transducers assigned to the rotation axes. The target point to be measured is provided with a retro-reflector (in particular corner cube prism or arrangement of three mirrors placed perpendicularly on one another), the retro-reflector reflecting the measurement beam of the laser tracker incident thereon back to the latter. The reflected measurement beam, in this case, travels coaxially with the emitted measurement beam when the measurement beam strikes the reflector exactly centrally, and with a parallel offset thereto when the measurement beam does not strike the reflector centrally. Depending on the embodiment of the tracker (absolute distance meter or interferometer), an absolute distance between the laser tracker and the target point and/or a change in this distance is deduced from a comparison of the emitted and reflected laser light. The position of the reflector, or of the target point, relative to the tracker is calculated from the angles recorded by the angle transducers and the distance detected by the distance meter.
A part of the reflected measurement beam is conventionally sent onto a PSD (position sensitive device). From the position at which the reflected measurement beam strikes the photosensitive surface of the PSD, the parallel displacement of the reflected measurement beam relative to the emitted measurement beam is deduced. The measurement data, thereby determined, define the parallel offset of the reflected measurement beam and are used for control of the measurement beam direction in such a way that the measurement beam follows the target point (tracking) when the latter moves. This means that corresponding variation of the measurement beam direction, or of the orientation of the mirror which aligns the measurement beam, is used to ensure that the parallel offset between the emitted measurement beam and the reflected measurement beam is reduced, or remains as small as possible.
Such a PSD has a small aperture angle for which reason, as disclosed for example in WO 2007/079600 A1, EP 2 071 283 A2 or WO 2009/046763 A1, a target detection unit may additionally be provided. The target detection unit comprises an image detection device having an image sensor, which moves with the measuring device and has a larger field of view than the optics used for the tracking with the PSD. If the tracker (or coordinate measuring device) loses alignment with the reflector, for example because the reflector has moved too rapidly, or because the measurement beam has been interrupted by an obstacle, the target detection unit can detect the reflector and realign the tracker with the reflector. To this end, the image detection device may have its own light source. It is also possible to configure a device without a PSD and only with (at least) one target detection unit.
Such a light source for an image detection device is selected as a laser diode for devices with sizeable measurement distances, for example in ranges of around 80 to 160 meters. In this way, the required light intensity of the reflected measurement light beam can also be maintained over such large distances. However, the problem arises that a plurality of transverse modes are excited in laser diodes. This gives rise to an asymmetrical, highly granular intensity distribution at the position where the light emerges from the laser diode or the light waveguide (granulation). This granular intensity distribution is imaged onto the image sensor and vitiates the position detection of the measurement light beam by the image sensor. The inaccuracy is further exacerbated when the retro-reflector only images a section of the illumination beam onto the PSD. The reason is that in this case the centroid of the light intensity is subject to strong variations not only due to a spontaneously varying energy distribution into the various modes, but also when the section is displaced, for example when the reflector moves.
Known means for counteracting this effect are, for example:                before emission, the light emitted by the laser diode is guided through a multimode fiber, which constitutes a scrambler that generates a predetermined distribution of the light energy over the modes.        decoherence of the emitted light is achieved by radio frequency modulation of the laser diode in the MHz to GHz range.        
In the coordinate measuring devices described in WO 2007/079600 A1, problems with granulation effects do not arise since the HeNe laser used in combination with a single-mode glass fiber corresponds to an ideal radiator having one wavelength and a symmetrical, gaussian-distributed beam profile. Owing to the symmetrical beam profile, displacements of the beam can be reliably detected and converted by means of a PSD into position information. The illumination beam is already collimated.